PARIS, Jan 11 (Reuters) - When the opponents of gay-marriagetake to the streets in Paris on Sunday, their protest will beled neither by politicians nor priests, but by a sassy comedianin a pink T-shirt who goes by the stage name Frigide Barjot.

With her on the march, expected to be one of the capital'sbiggest demonstrations in years, will be a young gay man whocampaigns against homosexual marriage and an an older activistfrom the right-to-life movement.

Notably absent will be most religious leaders who set thetone for the opposition with talking points based on social andlegal arguments rather than appeals to faith.

"We're all born of a man and a woman, but the law will saythe oppositite tomorrow," says Barjot, warning the reform wouldbreak links between father, mother and children that groundhuman society. "It will say a child is born of a man and a man."

Cardinal Andre Vingt-Trois, head of the Catholic Churchhere, will go to greet some protesters but not march. ChiefRabbi Gilles Bernheim, whose position paper won praise even fromPope Benedict, has no plan to join the demonstration either.

Asked if the hierarchy could take over the movement, sheprotested that she was in charge. "If I'm the Catholic Church,the Vatican will have to review its code of canon law," shesaid. "I'm not a bishop and I'm not ready to become one."

Organisers have declined to estimate the size of the crowd,expected in the hundreds of thousands, but say about 900 busesand five trains have been reserved to bring marchers from theprovinces to join the Paris protest.

REFORM ASSURED BUT SUPPORT SLIPPING

The mixed bag of organisers behind the "Demo for All" - atake-off on the reform slogan "Marriage for All" - surprisedPresident Francois Hollande's left-wing government by rallyingabout 100,000 for its first protest last November.

The government's comfortable majority in parliament ensuresit can pass the law by June as planned and so make good on anelection pledge by Hollande.

Same sex marriage is already legal in 11 countries,including Belgium, Portugal, the Netherlands, Spain, Sweden,Norway and South Africa.

But the protests have already begun to shape the reform.Socialist deputies wanted the law legalising same-sex marriageand adoption to include the right for lesbians to use assistedprocreation methods such as artificial insemination.

But they decided this week to leave this option out afterseeing support for the main changes dropping in opinion pollssince the opposition campaign started last August.

Support for gay marriage has fallen from around 65 percentto the mid to low 50s in the latest polls, while the number ofthose backing adoption rights has slid below 50 percent.

This is at least partly due to the opponents' campaign stressing negative effects they see for families and childrenfrom same-sex marriage rather than the government's defence thatthe reform is simply an extension of equal rights to gays.

Barjot, a believer who published a book entitled Confessionsof a Trendy Catholic, sports the slogan "A child is born of aman and a woman" across her tight pink T-shirt.

SEX NOT GENDER

Politicians from the conservative opposition UMP and thefar-right National Front have said they would join the threemarches departing from different parts of Paris and convergingfor a final rally at the Eiffel Tower.

Barjot said they could join but only banners and postersdesigned for the march could be displayed.

"We've been very firm with groups and political parties thatwe don't want their slogans or banners," Barjot said, stressingthey especially wanted to avoid anti-gay rhetoric.

"We're marriagophile, not homophobe," she quipped.

She also opposed the gender theory - that sexual identity isa social construct rather than a product of nature - that liesbehind the argument that marriage can be both for heterosexualsand gays. "We want sex, not gender," she declared.

Lyon Cardinal Philippe Barbarin and a few other bishops planto march, but not as part of any official Church delegation.Much more important have been lay Catholic networks aroundFrance that have mobilised people to attend the march.

"Obviously the Catholic Church is participating in anexplicit and undeniable way, but it is not an organiser," saidChurch spokesman Monsignor Bernard Podvin.